In a typical vacuum cleaner, Vacuum cleaners are designed to separate dirt and dust from an airflow. In a typical vacuum cleaner, the motor and fan unit generates an airflow which draws dirt- and dust-laden air into the vacuum cleaner through a dirty air inlet. The airflow then passes through a form of separating apparatus to remove dirt and dust from the airflow. Some vacuum cleaners make use of a porous bag through which the dirty air is sucked so that the dirt and dust is retained in the bag whilst cleaned air is exhausted to the atmosphere. In other vacuum cleaners, cyclonic separators are used to separate dirt and dust from the airflow. The cleaned air is then drawn through a filter and the motor and fan unit itself. The air exiting the motor and fan unit may pass through another filter arranged to remove small carbon particles produced by the motor itself and/or allergens and other microscopic particles, before exiting the vacuum cleaner through an exhaust.
A problem which may be encountered with such appliances is that of tones generated by the rotating fan. As the blades of the fan rotate, they cause pressure fluctuations in the surrounding air and effect a tonal noise at a blade passing frequency (BPF). The BPF is proportional to the rotational speed of the fan and the number of blades of the fan. Such a fan tone can be intense and annoying for a user of the appliance.